I agree with those who said the car has enough power to get a load up to speed. I wouldn't be terribly concerned about the brakes, either. My worry would be trying to brake while in any sort of curve and the traction/handling issues. Part of the beauty of the Mirage is that they stripped away most of the excess weight, but that doesn't leave a huge margin of extra capacity either.
My bicycle can tow a trailer or a canoe, but it does handle differently and I wouldn't want to find myself trying to handle that load going down a mountain.
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View my fuel log 2014 Mirage SE 1.2 manual: 45.0 mpg (US) ... 19.1 km/L ... 5.2 L/100 km ... 54.1 mpg (Imp)